The field of art to which this invention pertains is pigments used in the coating of cellulose-based printing media.
Coating pigments based on swellable smectic clays are known from European Patent Application No. 283,300. In addition to smectic clays, such coating pigments can also contain up to 30 percent secondary pigments or extender pigments, such as kaolin or calcium carbonate. The amount of pigment applied is no more than 5 g/m.sup.2, preferably no more than 1 g/m.sup.2. An example of a smectic clay that can be used is naturally occurring sodium bentonite (Wyoming bentonite). However, this clay has a swelling capacity of 50 ml (2 g in 100 ml water). Despite its high swelling capacity, its adhesion to paper is very poor without a binder which is why printability by the offset printing method (water contact) is problematical. The use of European bentonites with exchangeable sodium and calcium ions has also been described. However, coatings produced with these bentonites also exhibit poor adhesion to the paper surface in the absence of binders. This effect is manifested especially by the phenomenon of "missing dots." Otherwise, these layer silicates and the use conditions are not defined in greater detail. Since these bentonites are natural products, the properties vary considerably, which is manifested especially by very poor handling in production and fluctuations in the capacity of the shine-and strike-through of the printing inks. In the case of sodium bentonites, the high swelling volume and thus the high viscosity prevent the application of the coating pigment under normal operating conditions (see also Tappi Journal, May, 1992, pages 231 to 237; Das Papier, Volume 16, 1992, pages 273 to 277). Papers coated with these pigments have a low pick resistance. Binders, such as starch or latex are added in almost all examples in order to improve adhesion to the paper fiber. A synergistic effect is also observed. These binders make surface finishing more expensive and also interfere with recycling and deinking of paper in reprocessing. They also increase the COD value of the processing water which is generally recycled, which in turn leads to increased susceptibility to problems in paper production (runnability). Thus there may be sticking to felts, wires, etc., or holes in the paper. In addition, the pigment coating and printing ink are fixed on the fiber in such a way that separation of the printing ink can be accomplished only with the help of large quantities of chemicals or it is associated with a great loss of fiber or a loss of whiteness.
German Patent 736,450 describes a process for production of coated paper using a coating composition that contains a binder and pigment wherein bentonite or a similar swelling clay is used as the binder in the coating compound. However, the material used in this process is not characterized further. Binders are normally used in amounts on the order of 15 to 25 percent so the bentonite content according to this patent would be in the same range. Furthermore, fixation of the pigments with the help of the inorganic binder of bentonite is not proven and the addition of conventional binders is mentioned.
German Patent Application 4,038,886 discloses water-based organophilic layer silicates based on bentonite for coating paper. The organophilic layer silicates are obtained by reacting sodium bentonite with quaternary ammonium salts in a ratio of 69:31. European Patent Application 192,252 describes a similar organophilic bentonite and its use in coating compositions based on organic solvents. Neither case, however, involves modification of the surface of the sodium bentonite in the sense of alternating the zeta potential but instead these two prior art documents concern processes for rendering the bentonite completely hydrophobic. Therefore these pigments can be processed to water-based coatings only by using additional organic wetting agents.
A similar reaction (layer silicates based on vermiculite and/or mica) with alkylammonium compounds is disclosed in German Patent 3,636,850. Films can be produced from aqueous suspensions of these organophilic silicates. However, these materials are completely unsuitable for the purpose of the present invention.
According to U.S. Pat. No. 4,943,324, paper fillers are obtained by treating the surfaces of kaolin pigments with an amine. The purpose of this invention is to prevent loss of strength between the paper fibers that occurs when using inorganic fillers.
Finally, U.S. Pat. No. 3,508,952 describes pretreatment of layer silicates with chromium salts and/or zirconium salts to reduce the gas permeability of cellulose-based substrates.
The object of this invention is to provide coating pigments for cellulose-based printing media that will impart improved printability, lower ink strike-through and better print opacity to the coated printing media. In particular, the coating pigments are suitable for application by modern coating equipment at high speeds, and a low coating weight is sufficient to achieve the desired effects (printability) so as to guarantee the possibility of producing paper of LWC and ULWC grades, for example, with a greatly reduced grammage (weight of sheet of paper in grams per square meter). Furthermore, this invention is directed to the problem of providing printing media that can be deinked easily and thus is "more environmentally friendly" than the printing media available in the past.